High energy product, high coercivity permanent magnet compositions comprising, for example, iron, neodymium and/or praseodymium, and boron and methods of making them are disclosed in U.S. Ser. Nos. 414,936 filed Sept. 3, 1982, 508,266 filed June 24, l983, now abandoned and 544,728 filed Oct. 26, 1983, all by John J. Croat and assigned to the assignee of this application. An illustrative composition, expressed in atomic proportions, is Nd.sub.0.l3 (Fe.sub.0.95 B.sub.0.05).sub.0.87. It is substantially the composition of a specific stable intermetallic phase that possesses high coercivity when formed as fine crystallites about 20 to 400 or 500 nanometers in largest dimension.
As disclosed in said U.S. Ser. No. 544,728, which is incorporated herein by reference, the essential and predominant (but not the sole) constituent of such permanent magnet compositions is a tetragonal crystal phase exemplified by the atomic formula Nd.sub.2 Fe.sub.14 B.sub.1. The length of the crystallographic c-axis of the tetragonal crystal is about 12.18 Angstroms, and the length of the a-axis is about 8.78 Angstroms. The phase can be identified more generally by the atomic formula (RE.sub.1-a RE'.sub.a).sub.2 (Fe.sub.1-b TM.sub.b).sub.14 B.sub.1 where RE is neodymium and/or praseodymium; RE' is one or more rare earth elements taken from the group consisting of yttrium, lanthanum, cerium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium; and TM is one or more transition metal elements taken from the group consisting of cobalt, nickel, manganese, chromium and copper; and a is from about 0 to 0.4 and b is from about 0 to 0.4.
Melts of the above family of compositions can be very rapidly quenched, such as by melt spinning, to produce a solid material, e.g., a thin ribbon. When the rate of cooling hasbbeen controlled to produce a suitable fine crystalline microstructure (20 nm to 400 or 500 nm,, the material has excellent permanent magnet properties. On the other hand, faster cooling (overquenching) produce a material with smaller crystallites and lower coercivity. However, as disclosed, such overquenched material can be annealed to form the suitable crystal size with the associated high coercivity and high energy product.
An interesting and useful property of this neodymium-iron-boron material (for example) is that it is magnetically isotropic. A fine grain, melt-spun ribbon can be broken up into flat particles. The particles can be pressed i a die at room temperature to form a unitary body of about 85 percent of the material's density Bonding agents can be employed before or after the compaction. The making of such bonded magnets is disclosed in U.S. Ser. No. 492,629, filed May 9, 1983, (now abandoned) by Robert W. Lee and John J. Croat and assigned to the assignee hereof. It was surprising to find that such bonded magnets displayed no preferred magnetic direction. Values of intrinsic coercivity or maximum energy product were not dependent upon the direction of the applied magnetic field. There was no advantage in grinding the ribbon to very fine particles and magnetically aligning the particles before compaction.
Such magnetically isotropic materials are very useful because they can be easily pressed (without magnetic alignment) into bonded shapes. The shapes can be magnetized in the most convenient direction.
It is recognized that the iron-neodymium-boron type compositions might provide still higher energy products if at least a portion of the grains or crystallites in their microstructure could be physically aligned and if such alignment produced at least partial magnetic domain alignment. The material would then have a preferred direction of magnetization. The material would be magnetically anisotropic and would have higher residual magnetizatio and higher energy product in the preferred direction. I have now accomplished this using overquenched-melt-spun material by hot working the material to consolidate it to full density and to effect plastic flow that yields magnetic alignment. The same improvement can be accomplished on finely crystalline, high coercivity material (e.g., H.sub.ci &gt;1000 Oe) if the hot work is performed before excessive grain growth occurs and coercivity decreases.
It is an object of my invention to provide a fully densified fine grain, anisotropic, permanent magnet formed by hot working a suitable material comprising iron, neodymium and/o praseodymium, and boron. This anisotropic magnet has higher residual magnetization and energy product than isotropic magnets of like composition.
It is an object of my invention to provide a method of treating overquenched compositions containing suitable proportions of iron, neodymium and/or praseodymium, and boron at suitable temperatures and pressures to fully densify the material into a solid mass, to effect the growth of fine, high coercivity crystallites add to cause a flow and orientation of the material sufficient to produce macroscopic magnetic alignment.
It is another object of my invention to treat suitable transition metal-rare earth metal-boron compositions that do not have peraanent magnet properties because their microstructure is amorphous or too finely crystalline. The treatment is by a hot working process, such as hot pressing, hot die upsetting, extrusion, forging, rolling or the like, to fully consolidate pieces of the material, to effect suitable grain growth and to produce a plastic flow therein that results in a body having magnetic alignment. It is found that the maximum magnetic properties in such a hot worked body are oriented parallel to the direction of pressing (perpendicular to the direction of flow). In the direction of preferred magnetic alignment, energy products are obtainable that are significantly greater than those in isotropic magnets of like composition.
It is also to be recognized that hot pressing for the purpose of consolidation to full density is beneficial even without substantial magnetic alignment.